Question

我需要的是这样一个简单易用的查询，它让我感到震惊的是我在LINQ中尝试做了多少工作。在T-SQL中，它将是：

SELECT I.InvoiceID, I.CustomerID, I.Amount AS AmountInvoiced,
       I.Date AS InvoiceDate, ISNULL(SUM(P.Amount), 0) AS AmountPaid,
       I.Amount - ISNULL(SUM(P.Amount), 0) AS AmountDue
FROM Invoices I
LEFT JOIN Payments P ON I.InvoiceID = P.InvoiceID
WHERE I.Date between @start and @end
GROUP BY I.InvoiceID, I.CustomerID, I.Amount, I.Date
ORDER BY AmountDue DESC

我提出的最佳等效LINQ表达式，花了我更长的时间：

var invoices = (
    from I in Invoices
    where I.Date >= start &&
          I.Date <= end
    join P in Payments on I.InvoiceID equals P.InvoiceID into payments
    select new{
        I.InvoiceID, I.CustomerID, AmountInvoiced = I.Amount, InvoiceDate = I.Date,
        AmountPaid = ((decimal?)payments.Select(P=>P.Amount).Sum()).GetValueOrDefault(),
        AmountDue = I.Amount - ((decimal?)payments.Select(P=>P.Amount).Sum()).GetValueOrDefault()
    }
).OrderByDescending(row=>row.AmountDue);

在针对SQL Server运行时，它会获得等效的结果集。但是，使用SQL CE数据库会改变一些事情。 T-SQL几乎保持不变。我只需要将ISNULL更改为COALESCE。但是，使用相同的LINQ表达式会导致错误：

There was an error parsing the query. [ Token line number = 4,
Token line offset = 9,Token in error = SELECT ]

所以我们看一下生成的SQL代码：

SELECT [t3].[InvoiceID], [t3].[CustomerID], [t3].[Amount] AS [AmountInvoiced], [t3].[Date] AS [InvoiceDate], [t3].[value] AS [AmountPaid], [t3].[value2] AS [AmountDue]
FROM (
    SELECT [t0].[InvoiceID], [t0].[CustomerID], [t0].[Amount], [t0].[Date], COALESCE((
        SELECT SUM([t1].[Amount])
        FROM [Payments] AS [t1]
        WHERE [t0].[InvoiceID] = [t1].[InvoiceID]
        ),0) AS [value], [t0].[Amount] - (COALESCE((
        SELECT SUM([t2].[Amount])
        FROM [Payments] AS [t2]
        WHERE [t0].[InvoiceID] = [t2].[InvoiceID]
        ),0)) AS [value2]
    FROM [Invoices] AS [t0]
    ) AS [t3]
WHERE ([t3].[Date] >= @p0) AND ([t3].[Date] <= @p1)
ORDER BY [t3].[value2] DESC

唉！好的，所以在针对SQL Server运行时它是丑陋而低效的，但是我们不应该关心，因为它假设更快地写，性能差异不应该那么大。但它只是不对抗SQL CE，它显然不支持SELECT列表中的子查询。

事实上，我在LINQ中尝试了几种不同的左连接查询，它们似乎都有同样的问题。甚至：

from I in Invoices
join P in Payments on I.InvoiceID equals P.InvoiceID into payments
select new{I, payments}

产生

SELECT [t0].[InvoiceID], [t0].[CustomerID], [t0].[Amount], [t0].[Date], [t1].[InvoiceID] AS [InvoiceID2], [t1].[Amount] AS [Amount2], [t1].[Date] AS [Date2], (
    SELECT COUNT(*)
    FROM [Payments] AS [t2]
    WHERE [t0].[InvoiceID] = [t2].[InvoiceID]
    ) AS [value]
FROM [Invoices] AS [t0]
LEFT OUTER JOIN [Payments] AS [t1] ON [t0].[InvoiceID] = [t1].[InvoiceID]
ORDER BY [t0].[InvoiceID]

也会导致错误：

There was an error parsing the query. [ Token line number = 2,
Token line offset = 5,Token in error = SELECT ]

那么如何使用LINQ在SQL CE数据库上进行简单的左连接？我在浪费时间吗？

Answer 1

您是否尝试过使用group by的查询表达式，更接近您的T-SQL版本？

var invoices =
    from I in Invoices
    where I.Date >= start && I.Date <= end
    join P in Payments on I.InvoiceID equals P.InvoiceID into J
    group J.Sum(p => p.Amount) by new { I.InvoiceID, I.CustomerID, I.Amount, I.Date } into G
    let AmountPaid = G.Sum()
    let AmountDue = G.Key.Amount - AmountPaid
    orderby AmountDue descending
    select new
    {
        G.Key.InvoiceID,
        G.Key.CustomerID,
        AmountInvoiced = G.Key.Amount,
        InvoiceDate = G.Key.Date,
        AmountPaid,
        AmountDue
    };

结果与内存中的集合相符：

var Invoices = new[] {
    new { InvoiceID = 1, CustomerID = 2, Amount = 2.5m, Date = DateTime.Today },
    new { InvoiceID = 2, CustomerID = 3, Amount = 5.5m, Date = DateTime.Today }
}.AsQueryable();
var Payments = new[] {
    new { InvoiceID = 1, Amount = 1m }
}.AsQueryable();

收率：

{ InvoiceID = 2, CustomerID = 3, AmountInvoiced = 5.5, InvoiceDate = 8/15/2009,
  AmountPaid = 0, AmountDue = 5.5 }
{ InvoiceID = 1, CustomerID = 2, AmountInvoiced = 2.5, InvoiceDate = 8/15/2009, 
  AmountPaid = 1, AmountDue = 1.5 }

如果这不起作用，LINQ左连接通常在连接结果上使用DefaultIfEmpty()。你可能不得不做这样的事情：

var invoices =
    from I in Invoices
    where I.Date >= start && I.Date <= end
    join P in Payments on I.InvoiceID equals P.InvoiceID into J
    from PJ in J.DefaultIfEmpty() // Left Join
    group PJ by new { I.InvoiceID, I.CustomerID, I.Amount, I.Date } into G
    let AmountPaid = G.Sum(p => p == null ? 0 : p.Amount)
    // etc...

LINQ聚合在SQL CE上保持连接

1 个答案: